Acrylic resin is excellent in transparency, and a system (core-shell type rubber, for example) to which acrylic rubber is added is excellent in processability, so that acrylic resin is widely used as films, sheets, and general molded articles.
Widely used in recent years as a method of decorating a surface layer of a resin molded article instead of paining are: a transfer method in which an acrylic resin film (hereinafter referred to as a “decorative laminated film”) decorated by printing or the like is inserted in an injection molding mold, injection molding is performed, only a decorative layer is transferred to the surface of a molded article, and the film is peeled off; a method, such as an insert molding method or an in-mold molding method, of performing decoration simultaneously with injection molding, that is, forming a decorative film on a molded article as an outermost surface of the molded article; and a method of laminating a film on the surface of an injection molding article. According to these molding methods, an excellent design and high-class feeling that are specific to the acrylic resin can be given to the molded article.
Further, light transmissive acrylic resin is widely used as a material constituting a film-shaped or plate-shaped optical member (such as a film, a substrate, or a prism sheet used in a liquid crystal display device) used in various optical related devices. Such resin is generally called “optical resin” or “optical polymer”, and a film made of such resin is called an “optical film.”
If the decorative laminated film or the optical film is used for interior and exterior materials of vehicles such as cars or for liquid crystal display devices such as personal computers and liquid crystal TVs, and extrusion molding by a T die method using a flat die is performed as a method of molding an acrylic resin film, a line-like defect called a die line continuously formed in a flow direction of the film. Thus, appearance design may deteriorate, and when the film is incorporated in the liquid crystal display device, display failures may be caused in practical use.
It is generally said that the die line is formed when a minute break, called a crack, or a chip is formed at a land portion or lip portion of a die or when resin, a decomposition product of resin, or the like sticks to the land portion or the lip portion to scratch molten resin.
These significantly limit the use of the acrylic resin film.
PTL 1 describes that: an adhered substance called gum becomes a cause of the formation of the die line in the extrusion molding using a T die; and to avoid the formation of the die line, a pull-down angle is adjusted.
Known as one of properties of the molten resin is a relaxation modulus. The relaxation modulus is a physical property that indicates the ease of disappearance of the deformation of the molten resin.
PTL 2 describes that by controlling the relaxation modulus of polypropylene-based resin, the viscosity of the resin in foam molding is improved, and therefore, a cell wall can be maintained. However, PTL 2 does not describe anything about the extrusion molding using the T die and does not describe the correlation between the relaxation modulus and the die line.